An SiC semiconductor is a semiconductor made of SiC (silicon carbide) which is a compound of carbon (C) and silicon (Si). A most distinctive feature of the SiC semiconductor is to have a physical value suitable as a material of semiconductor devices (power devices) used in power electronics. For example, in the case of commercially available single-crystal 4H-SiC, a band gap is 3.3 eV which is three times as wide as conventional Si-semiconductors, dielectric breakdown field strength is 3 MV/cm which is about ten times as high as conventional Si-semiconductors and saturation electron velocity is three times as fast as conventional Si-semiconductors. Further, SiC-semiconductors are better in thermal conductivity, heat resistance and chemical resistance than conventional Si-semiconductors and also have a feature of having a higher radiation resistance than Si-semiconductors. From these features, SiC-semiconductors, particularly MISFETs (MIS field-effect transistors) or MOSFETs (MOS field-effect transistors) of SiC are preferably used for semiconductor devices used in power electronics.
However, it has been conventionally problematic that there are many defects in an interface between a gate insulating film (gate oxide film) and SiC and channel mobility is low in MIS(MOS)FETs of SiC. Particularly, electron mobility in a bulk crystal is as high as 800 to 1000 cm2/Vs in 4H-SiC, whereas it has been a problem that the channel mobility (Si-face) of MIS(MOS)FETs of SiC is as low as 10 cm2/Vs.
Further, conventionally, an insulating film has been formed by thermal oxidation of SiC or using a CVD method and an interface between the formed insulating film and SiC has been nitrided such as by NO, N2O or NH3 gas, thereby reducing defects in the interface to improve the channel mobility. However, the channel mobility (Si-face) of the MIS(MOS)FET of SiC is as low as 40 to 50 cm2/Vs even if the interface is nitrided and a further improvement in the channel mobility is much-needed.
The channel mobility of the MIS(MOS)FET of SiC is low because there are many defects in the interface of SiC produced by conventional technologies, i.e. interface state density is high. Due to the low channel mobility of the MIS(MOS)FET of SiC, an on-resistance value of the MIS(MOS)FET transistor increases. If the on-resistance value of the transistor increases, power consumption increases.
As described above, despite the fact that the electron mobility in the bulk crystal is originally as high as 800 to 1000 cm2/Vs in 4H-SiC, the channel mobility is reduced due to defects (magnitude of the interface state density) if SiC is incorporated in devices such as a MIS(MOS)FET. That is, by device integration, the potential of SiC originally having a high electron mobility cannot be utilized at all.
Thus far, there have been known many prior arts for reducing an on-resistance of a transistor by improving the channel mobility of a MIS(MOS)FET. For example, there is known a manufacturing method for improving channel mobility by lower density or n-type conversion of a surface of a p-type layer through ion implantation of nitrogen or phosphorus into SiC immediately below a gate insulating film or epitaxial growth (for example, see patent literature 1). There is also known a manufacturing method for improving channel mobility by nitriding an interface between an insulating film and SiC (for example, see patent literature 2). There is further known a manufacturing method for improving channel mobility by performing a heat treatment on a gate insulating film (gate oxide film) in an atmosphere containing H2O (for example, see patent literature 3).
However, the above patent literature 1 has a problem that reliability of the insulating film provided on an SiC substrate is poor since nitrogen or phosphorus is ion-implanted into SiC immediately below the gate insulating film or the epitaxial growth is allowed. There is also a problem in controllability of impurity concentration and film thickness of epi-layer. Resulting from that, there is also a problem that the channel mobility and a threshold voltage vary.
Further, in the above patent literature 2, the channel mobility is about 40 cm2/Vs as described above although the interface state density is reduced and the channel mobility is improved by nitriding the interface. Further, in the above patent literature 3, the channel mobility is about 50 cm2/Vs although the channel mobility is improved by performing the heat treatment on the gate insulating film (gate oxide film) in the atmosphere containing H2O. There is also a problem that an effective range of a process condition is narrow.    [Patent Document 1] JP 2000-150866 A    [Patent Document 2] JP 2006-216918 A    [Patent Document 3] JP 2003-086792 A